Pre tech education

[nimoh]

Well, since two matched divers with RMV's of 0.4 cuft/min can both use their own single 80 (77.4) air filled to 3200 psi to do a 20 minute true buddy dive to 100' and hit real deco while observing ALL protocols from thirds to rock bottom and a 3 minute safety stop, who am I to say it is a bad idea?

(I wouldn't do it either)

Can you clarify what is meant by a "true buddy dive"?
My guess is you mean that both divers always have enough gas to get to the next switch, but not sure.

I am also not sure what you mean by "real deco"?
20 minutes at 100' is right about at the NDL

Lastly, by my calculations, a 0.4 diver going to 100' for 20 minutes would use (0.4 * 4 * 20) 32cuft of gas which is beyond thirds. Not saying you would need to use thirds on this dive, but you did point out that they would be observing thirds. I would also tend to agree with Tom that the 0.4 shouldn't be used for planning since in an emergency the breathing rate would go up even for experienced divers, let alone someone taking their first tec class.

edit: when I say "0.4 shouldn't be used for planning..." I am basing this on the assumption that 0.4 is a resting rate, not a working rate but concede that for many divers, 0.4 could be a working rate.

 

[Kevrumbo]

Some simple thoughts

Learn to round down to the closest number easily divisible by three, then divide by three, and subtract the result from the total (unrounded value). It's a recipe for longevity (this is not a joke). Even better, divide by four. Turn around pressure.

Other usefull knowledge includes the multiplication tables of 0.21, 0.28, 0.32, 0.36, 0.40 and 0.50. Dividing 1.4, 1.5 or 1.6 by those numbers is popular, too. Basic nitrox calculations.

Learn to calculate your air consumption rate (to estimate your available bottom time) and learn to use some dive planning software.

Simple things like these are important. Deeper understanding of physiology is fun, but doesn't always help at the pond, unless it is a very very deep pond.

Easier just to convert over to the Metric System, especially if you're going to do a lot of travel diving overseas outside the US -and buddying up with European & Asia Divers.

Easy imperial US/metric conversions for depth & pressure, that you can do in your head for now:

Depth in meters multiplied by 10/3 gives depth in feet;
Feet multiplied by 3/10 gives meters.
Example: 18m(10/3) = 60' ; 60'(3/10) = 18m

Pressure bar multiplied by 3/2, and multiplied again by 10 gives pressure psi;
Pressure psi multiplied by 2/3, and divided by 10 gives pressure bar.
Ex): 200bar(3/2)(10) = 3000psi ; 3000psi(2/3)/10 = 200bar.
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Your common Dive Table counting numbers, or reference cardinal numbers, for depth in scuba are:

Imperial US (feet) by 10's:
Ex): 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 etc

Metric system goes by 3's:
Ex): 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33 etc

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One of the major reasons it's easier to do Gas Consumption arithmetic in metric is figuring out your depth in Atmospheres Absolute (ATA): simply divide depth in meters by 10 and add 1. For example, 24 meters is 3.4 ATA (divide 24 meters by 10, add 1 equals 3.4 ATA). Try doing that as quickly & intuitively on-the-fly in Imperial/US Units: 80 feet divided-by-33 plus 1?

This critical value of ATA is your multiplying factor for your Surface Consumption Rate (also known as SAC Rate in litres/min), to figure out your actual gas consumtion rate at your target working Depth (also referred to as DCR or Depth Consumption Rate).

 

[lowviz]

Can you clarify what is meant by a "true buddy dive"?

Yep, the calcs say you can actually save your buddy at any time.

... I am also not sure what you mean by "real deco"?

Most, if not all, ascent profiles promoted your safety stop to a hard overhead.

Took a bit of figuring because the claim seemed so "on the very edge". I just wanted to satisfy my own suspicions. But then I just had to go and point out that it CAN be done.

That is why I'm a regular here, I have no self-control topside. Nothing more, I still wouldn't do a dive like that...

 

[RJP]

RJP, Tec40 is not an intro to tech course...intro to tech courses don't teach limited deco, they teach configurations and skills.

If I wasn't on my phone I would gladly pick apart your entire post, but I am not willing to spend the time doing it right now. Maybe later.

Lowviz, anyone planning a dive with a .4 working rate is a numbskull...

Ultimately, I agree that tech diving needs/means doubles. But I don't think that allowing someone to dive a single tank in an intro-level* course such as Tec 40 is going to cause the world to end... the theory, skills, etc all work diving a single tank or double. (I managed to do a planned 'light deco' dive on the Hilma Hooker recently with a back-mounted 80 and another-one slung.) Neither do I think that anyone would finish that course and assume they were ready to dive the U-869 on a single AL 80. Lest anyone continue to think that PADI actually "advocates" tech diving on a single tank... you can rest easy knowing that Tec 45 and up requires doubles.

*"The PADI Tec 40 course is where you transition from recreational scuba diving to technical diving. It’s a great place to start because it’s the first subdivision of the full PADI Tec Diver course and bridges the gap between no stop diving and full technical deep decompression diving. You gain experience and begin building the knowledge and skills you need to continue your tec diver training."

 

[decompression]

So, again I'll ask, how are TEC 40 students trained to determine sufficient gas supply and appropriate redundancy?


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[RJP]

So, again I'll ask, how are TEC 40 students trained to determine sufficient gas supply and appropriate redundancy?


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In the usual fashion... the presence or absence of a manifold doesn't change any of the calculations or planning.

 

[decompression]

RJP, this is my point. The student is taught thirds or rock bottom (not sure what "the usual fashion" is), it will take x cuft to complete our dive safely. As we only have a single 80 and a 40 deco bottle we either ignore gas planing or we don't dive the required depth/time. Have you run into this issue? I'm going to cut some profiles to see how likely this is (again, all I've ever taught is doubles), what are the depth and time requirements for the TEC 40 dives? A few have been done above but I'll do some for my info. This isn't so much an agency thing as I'm genuinely concerned and hope we pass on useful info to anyone that follows. Certainly the OP.

I never mentioned anything about a manifold, although I personally feel diving independent doubles while experiencing all the task loading at this level is less than optimal.


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[Seya]

Sorry I was wrong discovery tec needs doubles tec 40 doesn't. And get this even tec 50 you don't need a back up mask. Good luck doing deco without a backup if you lose your main or doing drills. Tec 40 doesn't have a minimum cylinder size unless it's a pony which needs to be 30cf

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[uncfnp]

And it looks like Padi isn't the only agency that has intro level, limited deco courses that allow single cylinder, dual outlet gear options. According to their website, so does IANTD with their Advanced Nitrox Diver course. The PADI course allow deco up to 10 minutes and max deco gas of EAN 50. I believe the IANTD course is 15 minute max deco time and mix up to 100%. Wonder if there are other agencies with similar options as well?

 

[RJP]

I never mentioned anything about a manifold, although I personally feel diving independent doubles while experiencing all the task loading at this level is less than optimal.


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My tongue-in-cheek point was that the planning and calculations necessary (whether you're planning thirds, rock-bottom, whatever) are the same whether you're diving doubles or a single tank:
- depth
- time
- gas supply
- gas consumption

Will using a single tank provide for an incredibly interesting or typical tech dive plan? No. Will it allow for a dive that's sufficient/appropriate for learning IN AN INTRO COURSE? Sure.

According to Baltic-lite a 100ft dive for 20min on air with an RMV of .7 yields a 10min deco obligation and requires 66cf of gas. Fits right into 1/3rds with a 100cf single... even in a lost gas scenario. (They may allow a single tank... but that doesn't mean it has to be an AL80.)

Is it ideal? Is it my preference? No. Does it work? Yup.